In a gas and steam turbine plant, the heat which is contained in the expanded working medium or hot gas from the gas turbine, is used for producing steam for the steam turbine. The heat transfer is carried out in a heat recovery steam generator which is connected downstream to the gas turbine and in which a number of heating surfaces for water preheating, for steam generation and for steam superheating, are customarily arranged. The heating surfaces are connected into the water-steam cycle of the steam turbine. The water-steam cycle customarily comprises a plurality of pressure stages, for example, three, wherein each pressure stage can have an evaporative heating surface.
For the steam generator which, as a heat recovery steam generator, is connected downstream on the hot gas side to the gas turbine, a plurality of alternative design concepts, specifically the design as a once-through steam generator or the design as a recirculating steam generator, are a possibility. With a once-through steam generator, the heating of steam generator tubes, which are provided as evaporating tubes, leads to an evaporation of the flow medium in the steam generator tubes in a once-through passage. In contrast to this, with a natural or forced recirculation steam generator the water which is guided in the cycle is only partially evaporated during a passage through the evaporating tubes. The water which is not evaporated in this case, after separation of the generated steam, is fed again to the same evaporating tubes for further evaporation.
A once-through steam generator, in contrast to a natural or forced recirculation steam generator, is subjected to no pressure limitation, so that it can be designed for live steam pressures far above the critical pressure of water (PKri≈221 bar), where no distinction between the water and steam phases, and consequently also no phase separation, is possible. A high live steam pressure promotes a high thermal efficiency and consequently low CO2 emissions of a fossil-heated power plant. Moreover, a once-through steam generator has a simple constructional form in comparison to a recirculating steam generator and consequently is producible at especially low cost. The use of a steam generator, which is designed according to the once-through principle, as a heat recovery steam generator of a gas and steam turbine plant, therefore, is especially favorable for achieving a high overall efficiency of the gas and steam turbine plant with a simple constructional form.
A heat recovery steam generator in horizontal constructional form offers special advantages with regard to production cost, but also with regard to necessary maintenance operations, in which heat recovery steam generator the heating medium or hot gas, that is the exhaust gas from the gas turbine, is guided in an approximately horizontal flow direction through the steam generator. Such a steam generator, which, with a design as a once-through steam generator with comparatively low structural and design cost, has an especially high degree of flow stability, for example is known from WO 2004/025176 A1. This steam generator has an evaporative once-through heating surface, which comprises a number of steam generator tubes or evaporating tubes which are connected parallel to the through-flow of a flow medium. In order to ensure in this case homogenization and stabilization of the flow conditions between evaporating tubes, which are arranged one behind the other when viewed in the hot gas direction, this once-through steam generator has a number of outlet headers which are connected downstream to the evaporative once-through heating surface and which with their longitudinal direction are oriented basically parallel to the hot gas direction, and so absorb the flow medium which flows from evaporating tubes which are arranged one behind the other, as seen in the hot gas direction, and which, therefore, are differently heated. These outlet headers of the evaporative once-through heating surface equally serve as inlet distributors for the downstream-connected superheater heating surface.
In general, a once-through steam generator is operated in low load mode or during starting with a minimum flow of flow medium in the evaporating tubes in order to ensure a safe cooling of the evaporating tubes and in order to avoid a possible steam formation in the economizer heating surface which is connected upstream on the flow medium side to the evaporative once-through heating surface. During starting or in low load mode, this minimum flow is not completely evaporated in the evaporating tubes, so that during such an operating mode still unevaporated flow medium is present at the end of the evaporating tubes. In other words, during this operating mode a water-steam mixture issues from the evaporating tubes. However, a distribution of such a water-steam mixture to superheater tubes, which customarily are connected downstream to the evaporating tubes, as a rule is not possible in the once-through steam generator; the distribution which is customarily provided rather presupposes that the flow medium which is to be distributed exclusively contains steam portions. Therefore, as a rule during starting or in low load mode of the once-through steam generator, a water-steam separation is necessary at the outlet of the evaporative once-through heating surface, which as a rule is carried out in so-called cyclone separators.
For design-related reasons, a through-feed of these cyclone separators with water is only possible to a limited degree. The heating surface which is usable for evaporation, therefore, has to lie upstream of the separators, as seen in the flow direction of the flow medium, and so is limited. This results in the live steam temperature being able to be controlled only within small limits by means of the feed water volume, wherein for a greater control range as a rule injection coolers are required. The limitation of the operational flexibility which is associated with these aspects, in addition to the high equipment cost, customarily as a rule gives rise to undesirably long starting times and reaction times during load changes of the once-through steam generator in low load mode.